Wankel engines can be classified by their geometric size in terms of radius (rotor center to tip distance, also the median stator radius) and depth (rotor thickness), and offset (crank throw, eccentricity, also 1/4 the difference between stator's major and minor axes). These metrics function similarly to the bore and stroke measurements of a piston engine. Displacement is 3√3radius·offset·depth, multiplied with the number of rotors (note that this only counts a single face of each rotor as the entire rotor's displacement). Nearly all Mazda production Wankel engines share a single rotor radius, 105 mm (4.1 in), with a 15 mm (0.6 in) crankshaft offset. The only engine to diverge from this formula was the rare 13A, which used a 120 mm (4.7 in) rotor radius and 17.5 mm (0.7 in) crankshaft offset.

Mazda rotary engines have a reputation for being relatively small and powerful at the expense of poor fuel efficiency. The engines became popular with kit car builders, hot rodders and in light aircraft because of their light weight, compact size, tuning potential stemming and inherently high power-to-weight ratio - as is true to all Wankel-type engine. Mazda put the engine into serial production, with NSU (Ro80) and Citroën (GS Birotor), as part of the Comotor joint-venture, between 1967 and 1977.

Wankel engines can be classified by their geometric size in terms of radius (rotor center to tip distance, also the median stator radius) and depth (rotor thickness), and offset (crank throw, eccentricity, also 1/4 the difference between stator's major and minor axes). These metrics function similarly to the bore and stroke measurements of a piston engine. Displacement is 3√3radius·offset·depth, multiplied with the number of rotors (note that this only counts a single face of each rotor as the entire rotor's displacement, and is of course incorrect as there are three faces, equivalent to three piston faces, per rotor, i.e. equivalent to a three-cylinder radial piston motor per rotor). Nearly all Mazda production Wankel engines share a single rotor radius, 105 mm (4.1 in), with a 15 mm (0.6 in) crankshaft offset. The only engine to diverge from this formula was the rare 13A, which used a 120 mm (4.7 in) rotor radius and 17.5 mm (0.7 in) crankshaft offset.[citation needed]

Wankel engines became common place in motor sport events, this created the problem of correcting the representation of each engine's displacement as provided by the manufacturer, for the benefit of competition. Rather than force the majority of participants (driving piston engine cars) to half their quoted displacement (likely resulting in confusion), most racing organizations simply decided to double the quoted displacement of Wankel engines.[1]

The key for comparing the displacement between the 4-cycle engine and the rotary engine is in studying the degrees of rotation for a thermodynamic cycle to occur. For a 4-cycle engine to complete every thermodynamic cycle, the engine must rotate 720° or two complete revolutions of the crankshaft. The rotary engine is different. The engine rotor rotates at 1/3 the speed of the crankshaft. On two rotor engines, front and rear rotors are 180° offset from each other. Each rotation of the engine (360°) will bring two faces through the combustion cycle (the torque input to the eccentric shaft). This said, it takes 1080° or three complete revolutions of the crankshaft to complete the entire thermodynamic cycle. Obviously, there is a disparity. How can we get a relatable number to compare to a 4-stroke engine? The best way is to study 720° of rotation of the two-rotor engine. Every 360° of rotation, two faces of the engine complete a combustion cycle. 720° will have a total of four faces completing their cycle. 40ci(654cc) per face times four faces equals 160ci or 2.6L. That's a well-reasoned number and now gives something that can be compared to other engines. In addition, since four faces passed by in the comparison, it’s like a four-cylinder engine. The 13B therefore compares well to a 2.6L 4-cylinder 4-cycle engine.[1]

By using the same formula, calculating actual displacement in which 1080° is the complete thermodynamic cycle of a rotary engine and a total of six faces completing their cycle, 40ci(654cc) per face times six faces equals 3924cc, 3.9L, or 240ci, in reference to a Mazda 13B rotary engine. "Each face has a swept volume of 40ci(654cc) and there are a total of six faces. With this known, the engine displacement should be 40ci(654cc) times six to equal 240ci(3.9L)." [1]

Mazda's first prototype Wankel was the 40A, a single-rotor engine very much like the NSU KKM400. Although never produced in volume, the 40A was a valuable testbed for Mazda engineers, and quickly demonstrated two serious challenges[citation needed] to the feasibility of the design: "chatter marks" in the housing, and heavy oil consumption. The chatter marks, nicknamed "devil's fingernails",[citation needed] were caused by the tip-seal vibrating at its natural frequency. The oil consumption problem was addressed with heat-resistant rubber oil seals at the sides of the rotors. This early engine had a rotor radius of 90 mm (3.5 in), an offset of 14 mm (0.6 in), and a depth of 59 mm (2.3 in).

The very first Mazda Cosmo prototype used a 798 cc L8A two-rotor Wankel. The engine and car were both shown at the 1963 Tokyo Motor Show. Hollow cast iron apex seals reduced vibration by changing their resonance frequency and thus eliminated chatter marks. It used dry-sump lubrication. Rotor radius was up from the 40A to 98 mm (3.9 in), but depth dropped to 56 mm (2.2 in).

One-, three-, and four-rotor derivatives of the L8A were also created for experimentation.[citation needed]

The 10A series was Mazda's first production Wankel, appearing in 1965. It was a two-rotor design, with each displacing 491 cc for a total of 982 cc. These engine featured the mainstream rotor dimensions with a 60 mm (2.4 in) depth.

The rotor housing was made of sand-cast aluminum plated with chrome, while the aluminum sides were sprayed with molten carbon steel for strength. Cast iron was used for the rotors themselves, and their eccentric shafts were of expensive chrome-molybdenum steel.[citation needed] The addition of aluminum/carbon apex seals addressed the chatter mark problem.

The first 10A engine was the 0810, used in the Series ICosmo from May, 1965 through July, 1968. These cars, and their revolutionary engine, were often called L10A models. Gross output was 110 hp (82 kW) at 7000 rpm and 130 Nm (96 lb·ft) at 3500 rpm, but both numbers were probably optimistic (rpm of the crankshaft).

The 10A featured twin side intake ports per rotor, each fed by one of four carburetor barrels. Only one port per rotor was used under low loads for added fuel economy. A single peripheral exhaust port routed hot gas through the coolest parts of the housing, and engine coolant flowed axially rather than the radial flow used by NSU. A bit of oil was mixed with the intake charge for lubrication.

The 0810 was modified for the racing Cosmos used at Nürburgring. These engines had both side- and peripheral-located intake ports switched with a butterfly valve for low- and high-RPM use (respectively)

The final member of the 10A family was the 1971 0866. This variant featured a cast-iron thermal reactor to reduce exhaust emissions and re-tuned exhaust ports. The new approach to reducing emissions was partly a result of Japanese Government emission control legislation in 1968, with implementation starting in 1975. Mazda called their technology REAPS (Rotary Engine Anti Pollution System)、 The die-cast rotor housing was now coated with a new process: The new Transplant Coating Process (TCP) featured sprayed-on steel which is then coated with chrome. Gross output was 105 hp (78 kW) at 7000 rpm and 135 Nm (99.5 lb·ft) at 3500 rpm.

Mazda began development on a single rotor engine displacing 360cc, and was being developed for kei car use in the Mazda Chantez but was never placed into production. A prototype engine is on display at the Mazda Museum in Hiroshima, Japan.[2]

The 13A was designed especially for front wheel drive applications.[citation needed] It had two 655 cc rotors for a total of 1310 cc. This was the only production Mazda Wankel with different rotor dimensions: Radius was 120 mm (4.7 in) and offset was 17.5 mm (0.7 in), but depth remained the same as the 10A at 60 mm (2.4 in). Another major difference from the previous engines was the integrated water-cooled oil cooler.

The 13A was used only in the 1969–1972 R130 Luce, where it produced 126 hp (94 kW) and 172 Nm (126 lb·ft). This was the end of the line for this engine design: the next Luce was rear wheel drive and Mazda never again made a front wheel drive rotary vehicle.

The 12A is an "elongated" version of the 10A — the rotor radius was the same, but the depth was increased by 10 mm (0.4 in) to 70 mm (2.8 in). Each of its two rotors displaced 573 cc for a total of 1146 cc. The 12A series was produced for 15 years, from May 1970 through 1985. In 1974, a 12A became the first engine built outside of western Europe or the U.S to finish the 24 hours of Le Mans.

In 1974, a new process was used to harden the rotor housing. The Sheet-metal Insert Process (SIP) used a sheet of steel much like a conventional piston engine cylinder liner with a chrome plated surface. The side housing coating was also changed to eliminate the troublesome sprayed metal. The new "REST" process created such a strong housing, the old carbon seals could be abandoned in favour of conventional cast iron.

Early 12A engines also feature a thermal reactor, similar to the 0866 10A, and some use an exhaust port insert to reduce exhaust noise. A lean-burn version was introduced in 1979 (in Japan) and 1980 (in America) which substituted a more-conventional catalytic converter for this "afterburner". A major modification of the 12A architecture was the 6PI which featured variable induction ports.

The ultimate 12A engine was the electronically fuel-injected engine used in the Japan-spec HB series Cosmo, Luce,[3] and SA series RX-7.[4] In 1982 a 12A turbo powered Cosmo coupe was officially the fastest production car in Japan.[citation needed] It featured "semi-direct injection" into both rotors at once. A passive knock sensor was used to eliminate knocking, and later models featured a specially-designed smaller and lighter "Impact Turbo" which was tweaked for the unique exhaust signature of the Wankel engine for a 5-horsepower increase.[4] The engine continued until 1989 in the HB Cosmo series but by that stage it had grown a reputation as a thirsty engine.

The 12B was a short lived engine that was produced for the Mazda RX-2, and the RX-3. It had increased reliability from previous series, and it introduced a single distributor. This was the beginning of the single distributor rotary engines, before that, the 12A and 10A were both twin distributor wankels. The improved 12B was quietly introduced in 1974.

The 13B is the most widely produced engine. It was the basis for all future Mazda Wankel engines, and was produced for over 30 years. The 13B has no relation to the 13A. Instead, it is a lengthened version of the 12A, having 80 mm (3.1 in) thick rotors. Each rotor had a displacement of 654 cc, for a total of 1308 cc displacement in the engine.

In the United States, the 13B was available from 1974 through 1978 and was then retired from sedans but continued in 1984 - 1985 RX-7 GSL-SE. It disappeared from the US market again in 1995, when the last US-spec RX-7s were sold. The engine was continually used in Japan from 1972's Mazda Luce/RX-4 through 2002's RX-7.

A tuned intake manifold was used in a Wankel engine for the first time with the 13B-RESI. RESI = Rotary Engine Super Injection. The so-called Dynamic Effect Intake featured a two-level intake box which derived a supercharger-like effect from the Helmholtz resonance of the opening and closing intake ports. The RESI engine also featured Bosch L-Jetronic fuel injection. Output was much improved at 135 hp (101 kW) and 180 N·m (133 lb·ft).

Like the 12A-SIP, the second-generation RX-7 bowed with a variable-intake system. Dubbed DEI, the engine features both the 6PI and DEI systems, as well as four-injector electronic fuel injection. Total output is up to 146 hp (109 kW) at 6500 rpm and 187 N·m (138 lb·ft) at 3500 rpm.

The 13B-T was turbocharged in 1986. It features the newer four-injector fuel injection of the 6PI engine, but lacks that engine's eponymous variable intake system and 6PI. Mazda went back to the 4 port intake design similar to what was used in the 74-78 13B. In '86-'88 engines the twin-scroll turbocharger is fed using a two-stage mechanically actuated valve, however on '89-'91 engines a better turbo design was used with a divided manifold powering the twin-scroll configuration. For engines manufactured between '86-'88 output is rated at 185 hp (138 kW) at 6500 rpm and 248 N·m (183 lb·ft) at 3500 rpm.

A twin-turbocharged version of the 13B, the 13B-REW, became famous for its high output and low weight.[citation needed] The twin Hitachi HT-12 turbos were operated sequentially, with only the primary providing boost until 4,500 rpm, and the secondary additionally coming online afterwards. Notably, this was the world's first volume-production sequential twin turbocharger system.[5] Output eventually reached, and may have exceeded,[citation needed] Japan's unofficial maximum of 280 DIN hp (206 kW) for the final revision used in the series 8 Mazda RX-7.

In Le Mans racing, the first three-rotor engine used in the 757 was named the 13G.

The main difference between the 13G and 20B is that the 13G uses a factory peripheral intake port (used for racing) and the 20B (production vehicle) uses side intake ports.

It was renamed 20B after Mazda's naming convention for the 767 in November 1987.

The three-rotor 20B-REW was only used in the 1990-1995 Eunos Cosmo. It was the world's first volume-produced twin-turbo setup. It was offered in both 13B-REW and 20B-REW form. It displaced 1,962 cc (119.7 cu in) (three 654 cc rotors) and used 0.7 bar (10 psi; 70 kPa) of boost pressure to produce 300 hp (224 kW) and 407 N·m (300 lbf·ft).

A version of the 20B known as the "R20B RENESIS 3 Rotor Engine" was built by Racing Beat in the U.S.A for the Furai concept car which was released on 27 December 2007. The engine was designed to run on E100 ethanol fuel.[citation needed]

The most prominent 4-rotor engine from Mazda, the 26B, was used only in various Mazda-built sports prototype cars including the 767 and 787B in replacement of the older 13J. In 1991 the 26B-powered Mazda 787B became the first Japanese car and the first car with anything other than a reciprocating piston engine to win the 24 Hours of Le Mans race outright. The 26B engine displaced 2622 cc and developed 700 hp (522 kW) at 9000 rpm. The engine design uses peripheral intake ports, continually variable geometry intakes, and an additional (third) spark plug per rotor.

The Renesis engine – also 13B-MSP (Multi-Side Port) – which first appeared in production in the 2003 Mazda RX-8, is an evolution of the previous 13B. It was designed to reduce exhaust emission and improve fuel economy, which were two of the most recurrent drawbacks of Wankel rotary engines. It is naturally aspirated, unlike its predecessors from the 13B range, and therefore slightly less powerful than Mazda RX-7's twin-turbocharged 13B-REW (255-280 hp).

The Renesis design features two major changes from its predecessors. First, the exhaust ports are not peripheral but are located on the side of the housing, which eliminates overlap and allows redesign of the intake port area. This produced noticeably more power thanks to an increased effective compression ratio; however, Mazda engineers discovered that when changing the exhaust port to the side housing, a buildup of carbon in the exhaust port would stop the engine from running. To remedy this, Mazda engineers added a water jacket passage into the side housing. Secondly, the rotors are sealed differently through the use of redesigned side seals, low-height apex seals and the addition of a second cut-off ring. Mazda engineers had originally used apex seals identical to the older design of seal. Mazda changed the apex seal design to reduce friction and push the new engine closer to its limits.

These and other innovative technologies[citation needed] allow the Renesis to achieve 49% higher output and reduced fuel consumption and emissions (the RX-8 meets LEV-II[citation needed]). The Renesis won International Engine of the Year and Best New Engine awards 2003 [7] and also holds the "2.5 to 3 liter" (note that the engine is designated as a 1.3 liter by Mazda) size award [8] for 2003 and 2004, where it is considered a 2.6 L engine, but only for the matter of giving awards. It only actually displaces 1.3 liters.[9][10] This is because a 1,3L 2-rotor wankel has the same number of/equivalent power cycles in one rotation as that of a 2.6L four-stroke piston engine. Finally, it was on the Ward's 10 Best Engines list for 2004 and 2005.

The Renesis has also been adapted for a dual-fuel use, allowing it to run on gasoline or hydrogen.[11][12]

All the Mazda rotary engines have been praised because of their light weight.[citation needed] The unmodified 13B-MSP Renesis Engine has a weight of 112 kg (247 lbs), including all standard attachments (except airbox), but without engine fluids (such as coolant, oil, etc.). Known to make 210-235 hp.

Also known as the Renesis II, made its first and only appearance in the Mazda Taikiconcept car at the 2007 Tokyo Auto Show, but has not been seen since. It features up to 300 hp, a lengthened stroke, reduced width rotor housing, direct injection, and aluminum side housings.[13]

Mazda was fully committed to the Wankel engine just as the energy crisis of the 1970s struck. The company had all but eliminated piston engines from its products in 1974, a decision that nearly led to the company's collapse.[citation needed] A switch to a three-prong approach (piston-gasoline, piston-Diesel, and Wankel) for the 1980s relegated the Wankel to sports car use (in the RX-7 and Cosmo), severely limiting production volume. But the company had continued production continually since the mid-1960s, and was the only maker of Wankel-powered cars when (the RX-8) was discontinued from production in June 2012 with 2000 RX-8 Spirit R models being made for JDM (RHD) market.

Though not reflected in the graph at right, the RX-8 was a higher-volume car than its predecessors. Sales of the RX-8 peaked in 2004 at 23,690, but continued to decline through 2011, when less than 1000 were produced. [14]

On November 16, 2011, Mazda CEO Takashi Yamanouchi announced that the company is still committed to producing the rotary engine, saying, "So long as I remain involved with this company... there will be a rotary engine offering or multiple offerings in the lineup." [15]

^In several countries where a car tax is based on cylinder displacement, rotary engines are taxed according to this displacement equivalency rule. You pay the same tax for a 2,6L piston engine as for a 1,3L rotary engine.